COVID-19 and HVAC Systems | Cleaning & Maintenance Management

Indoor air quality is extremely important to the productivity and health of residents in any industrial, commercial or residential property.

HVAC air transport systems, with associated piping and ductless mini-split systems, ensure acceptable indoor air quality. However, they must be routinely inspected, maintained and cleaned to operate as efficiently and cost-effectively as possible. This routine includes cleaning the inside of the ductwork and replacing filters.

Air transport systems filter dust, particles, volatile organic compounds (VOCs), mold spores, bacteria and viruses. These systems also lower humidity while removing moisture in a building. Unfortunately, dust, particles, VOCs, mold spores, bacteria and viruses can accumulate on the inner surfaces of air transport systems and indoor ducts. This accumulated waste can be atomized and transferred via air currents.

Surface mounted HVAC system duct

Channel with accumulation of particles.

Virus transmission studies

Regarding SARS-CoV-2 transmission, a 2020 Florida Atlantic University study showed that a human sneeze or cough can disperse smaller droplet nuclei in air currents up to 12 feet away and linger in the air for more than a minute. A study conducted at an MIT lab using high-speed cameras and light confirmed that a fine mist of saliva and mucus can erupt from a person’s mouth at nearly 100 miles per hour and travel up to 27 feet.

Research suggests it is possible that droplets of the coronavirus are aerosolized, transported and transmitted through HVAC duct systems.

A ducted HVAC system creates air currents, which can likely carry smaller viral particles even further. A research letter published in the U.S. Centers for Disease Control and Prevention (CDC) Journal of Emerging Infectious Diseases in July 2020 detailed a COVID-19 outbreak related to air conditioning at a restaurant in Guangzhou, China. Ten people from three families who ate in the restaurant became infected. Atomized droplet transmission was caused by air-conditioned ventilation. A video from the restaurant showed that many of these customers were more than two meters apart. Researchers concluded that a strong flow of air from the air conditioner could have spread droplets from one table to another and back.

Research published on In July 2020, an ongoing environmental contamination study detailed surface and air samples obtained in two Nebraska Biocontainment Units (NBUs) and nine residential isolation rooms housing individuals who tested positive for SARS-CoV-2. The room surfaces tested include ventilation grilles, table tops and window frames. Virus samples with the highest concentration were recovered from an air handling grid in the NBU.

Airflow in the NBU suites came from a register at the top center of the room and exited through grilles at the head of the patient’s bed on either side of the room. Airflow modeling suggested that some of the airflow was directed under the patient’s bed, which may have caused the observed under-bed contamination, while the dominant airflow likely carried particles away from the patient’s bed to the edges of the room , along the windows , resulting in some deposition there.

The New England Journal of Medicine published a letter to the editor in March 2020 from a group of researchers studying the stability of SARS-CoV-2 in aerosols and on various surfaces. Their research found that infectious aerosols can last for several hours and on surfaces for as long as two days. The most long-lasting viability of viruses was on stainless steel and plastic. The estimated median half-life of SARS-CoV-2 was approximately 5.6 hours on stainless steel and 6.8 hours on plastic. In combination, influenza viruses can spread through the air on dust, fibers and other microscopic particles, according to Research published in August 2020 from the University of California, Davis and the Icahn School of Medicine at Mt. Sinai.

Accumulation of particles on a register

Accumulation of particles on a register

A look at the airflow

Although the duct interior can be made of smooth metal, fine dust can accumulate on the air duct surfaces, vents, motors and coils. Flakes of dead skin, hair and pet dander become trapped in the dust and before long this process creates a dense, mat-like environment where viruses, bacteria, mold, dust mites and allergens can thrive. This is similar to the exterior of a vehicle during a spring pollen season, as the pollen would cover the vehicle and windows.

For example, when dust is entrained by air currents generated by air conditioning and similar appliances, the dust acquires a positive or negative static electrical charge when it comes into contact with various objects. A substance with a positive electric charge is attracted to objects with a negative electric charge and vice versa. The greater the amount of dust in the air, the greater the amount of dust that clings to objects in the room.

If sources of dust (mainly people and clothing) are electrically charged, the dust generated by these sources also becomes electrically charged. This attraction generated by static electricity is known as the “Coulomb force.” In addition, air currents in the air exhaust ducts contain a high degree of humidity and humidity. This allows this moisture to adhere to the inside of the channels and collect dust particles.

HVAC air filters can help capture the viral particles and dust particles. A high-efficiency particulate air (HEPA) filter effectively removes small particles, 99.7% of 0.3 microns in size. A MERV (Minimum Efficiency Reporting Value) filter removes larger particles, 0.3 microns to 10 microns. A MERV filter of 17 or higher is considered HEPA-like in efficiency. However, most commercial buildings have MERV filters of 12 or lower. Unfortunately, a coronavirus particle is about 0.1 microns in size and cannot be viewed with human eyesight. So there is no guarantee that a HEPA filter will stop viral particles and dust particles.

Today’s standard air handling units (AHUs) in HVAC systems circulate up to 80 to 90% of the air in regular systems during peak loads, such as winter and summer, when the airflow from the outdoor ventilation is set to the minimum percentage to save energy. Standard filtration units in HVAC systems cannot effectively remove the virus in an airstream. The HVAC system has become a central point of contact to spread the virus by recirculating contaminated air in the room.

Based on the aforementioned research, inspection, testing, surface and air sampling, cleaning and disinfection of HVAC air transport systems and associated piping can help prevent the transmission of SARS-CoV-2.

Particles on the back of the register

Particles on the back of registers

Best Practices for Cleaning and Disinfecting HVAC System and Ducts

  1. Review HVAC air transport systems and associated duct drawings and diagrams.
  2. While donning appropriate Personal Protective Equipment (PPE), visually inspect the exterior and interior of the HVAC air transportation system and associated ductwork, registers/vents, motors, coils, and filters. If necessary, use a video borescope camera and take digital photos. Document and log any observations, even if the pipework “looks clean”.
  3. Test and inspect for air leaks, VOCs, particulates and metal seam problems on ductwork.
  4. Run samples before and after the surface of the HVAC air transport systems and associated ductwork, registers/vents, motors, coils and filters. The front surface sampling can identify any hazardous substances and hazardous particles.
  5. Perform pre- and post-air sampling of the project area. These samples help identify and isolate specific systems and affected areas, along with any hazardous materials and/or particulates.
  6. Use environmental controls and infection control risk assessment protocols to contain and isolate the cleaning and disinfection project. This includes setting up HEPA air scrubbers, negative air machines and containment barriers using a particle counter and air pressure monitor.
  7. Implement ultra-low volume (ULV) nebulization using a non-acidic, non-chlorine-based US Environmental Protection Agency (EPA)-registered virucidal, biocide and fungicidal disinfectant outside and inside ductwork, on registers/vents, motors, coils, and the entire project area, to inactivate any viral aerosols. For efficacy, observe a residence time of at least 10 minutes on surfaces or follow the residence time according to the virucidal label.
  8. Clean the internal duct surfaces of larger dust and particles, using HEPA vacuuming, rotobrushing, air tapping and mechanical cleaning. Fiberglass lined inner ducts require specialized attention and cleaning to prevent fiber damage and loosening. Visually inspect to avoid recontamination.
  9. Clean and disinfect on metal interior ducts, registers/vents, motors and coils with a non-acidic, non-chlorine based, EPA registered virucidal, biocide and fungicide disinfectant. For efficacy, observe a residence time of at least 10 minutes on surfaces or follow the residence time according to the virucidal label.
  10. Replace and install new HEPA filters. Ductless mini split systems may have a washable filter. It is best to vacuum the air transport unit (with HEPA filter) and to disinfect the washable filter often with an EPA registered virucidal, biocide and fungicide disinfectant. For efficacy, observe a residence time of at least 10 minutes on surfaces or follow the residence time according to the virucidal label.

Remember, it’s important to inspect, maintain, and clean HVAC systems on a routine schedule. Doing so will help your air transportation system operate at its most efficient, contributing to a healthier indoor air environment and saving operating costs over time.

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